Tandem furnace crystal growing device

ABSTRACT

Desired conductivity type and carrier concentration in compounds in which at least one constituent is volatile can be obtained by varying the partial pressure of one of the volatile constituents with respect to the compound melt. Thus the electrical and optical properties of the Group II-VI compounds such as the sulfides, selenides and tellurides of zinc, cadmium, and mercury, the Group III-V compounds such as the arsenides, phosphides, and antimonides of gallium, indium, and aluminum, and the Group IV-VI compounds such as the sulfides, selenides, and tellurides of germanium, tin, and lead are obtained by varying the partial pressure of one compound constituent with respect to the melt.

O United States Patent 1 1 1111 3,870,473

Kyle Mar. 11, 1975 [54] TANDEM FURNACE C S GROWING 3,628,998 12/1971Blum 23/301 3,690,847 9/1972 Merkel et al. 23/294 [75] Inventor: NanseR. Kyle, Long Beach, Calif. [73] Assignee: Hughes Aircraft Company.Primary Yudkoff n Ass/stun! Exammer--R. T. Foster Culver C1ty, al1f.

' Attorney, Agent, or Fzrm-James K. Haskell; Lew1s B. {22] Filed: Oct.30. 1972 Sternfels [2]] Appl. No.: 302,317 ABSTRACT Related ApphcauonData Desired conductivity type and carrier concentration in [62]Division of Ser. No. 69,025, Sept. 2, 1970, compounds in which at leastone constituent is volatile abandoned. can be obtained by varying thepartial pressure of one of the volatile constituents with respect to thecom- [52] Cl 23/273 23/294 SP pound melt. Thus the electrical andoptical properties t t of the Group Compounds Such as the u fides FleldOf Search SP, SP, Selenides and teuurides of i Cadmium and men 23/305294 cury, the Group lIlV compounds such as the arsenides, phosphides,and antimonides of gallium. in- [56] References cued dium, and aluminum,and the Group lV-Vl com- UNITED STATES PATENTS pounds such as thesulfides, selenides, and tellurides of 3,235,339 2/1966 Brunet 23/273germanium, tin, and lead are obtained by varying the 3,362,795 1/1968Wcishcck... 23/301 partial pressure of one compound constituent with re-3.48(l 394 l l/l969 MCl'kCl Cl. ill. spect to the melt 3,520,810 7 1070Pluskctt et ul. 23/305 3.00 137 9/1071 lnflgllChi 23 301 4 Clams, 4Drawmg Flgures L/JE 1 1 Q J j J///4 I230 a4 i l I l -za /a/ v4 1 d l l ll /7 1 4 l a 1 1 a 24 -fiw PATENTEU KARI 1 1975 saw 1 0f 2 Era Z Em z.

PATENTEU MRI 1 I975 sum 2 or g TANDEM FURNACE CRYSTAL GROWING DEVICEThis is a division, of application Ser. No. 69,025, filed Sept. 2, 1970,and now abandoned.

The present invention relates to a method and apparatus for obtainingdesired conductivity type and carrier concentration as well as opticalproperties in the Group ll-Vl, Group llI-V, and Group lV-Vl compoundsand, in particular, for synthesizing such compounds of a p-type, n-type,or intrinsic conductivity. Since no two compounds have exactly the sameelectrical, optical, or physical properties, a specific discussion ofthe properties of one compound does not necessarily apply directly toany other compound. For example, the melting point, vapor pressure, andequilibrium conditions for cadmium telluride cannot apply to any othercompound. However, cadmium telluride is exemplatory of the inventivemethod hereof.

The Group ll-Vl compounds, for example, have found extensive use assemiconductors, radiation detectors, optical modulators, and photosensitive devices. One compound of current interest is cadmiumtelluride. Much investigation has been conducted into this field,especially with respect to obtaining high resistivity compounds havingat least a resistivity of ohm-centimeters.

A foremost researcher in this field, D. de Nobel, in his PhillipsResearch Reports Vol. 14, pages 36l-492 (1959) (see also US. Pat. No.3,033,791) has shown that cadmium telluride can undergo stoichiometricdeviations and that such changes in composition are caused mainly bylattice defects which determine the conductivity and type of conduction.Since cadmium telluride does show a deviation from stoichiometry, theconstituents of the crystal itself can act as impurities. De Nobelsprocess utilizes a two step growth and diffusion or equilibratingtechnique under an atmosphere of one of the components in order toobtain the material with the desired electrical properties. Inparticular he first grew the cadmium telluride material, then cut itinto small rods and re-encapsulated the rods in vacuum with excesscadmium or tellurium, and finally subjected the encapsulated materialsin a furnace to equilibrate the cadmium telluride. A donor, such asindium, was utilized to make less critical the required pressure abovethe sample to obtain high resistivity in the cadmium telluride material.

The present invention also recognizes that cadmium telluride can undergostoichiometric deviations and such changes in composition are causedmainly by lattice defects which determine the conductivity and type ofconduction. The composition of cadmium telluride is a function of thechemical potential of one of the components when grown from the melt;therefore, it is desirable, as further recognized by the presentinvention, not simply to prevent decomposition of the melt, but to growcrystals of cadmium telluride under conditionos where the chemicalpotential of one of the components is controlled by providing a vapor ofone component or constituent above the melt. The temperaturecompositionphase diagram for such a compound can be graphically represented for atwocomponent system (A-B) with one compound, AB, showing deviation fromstoichiometry. For each point of the liquidus there is a correspondingpressure of the volatile component. If a particular pressure of thecomponent vapor is maintained over the melt, the system has a tendencyto remain at the composition corresponding to the vapor pressureapplied. As solidification takes place, the segregation process tends tomake the melt more and more concentrated with regard to the componentpresent in excess; however, the liquid is no longer in equilibrium withthe vapor, and a reaction between the vapor-melt occurs until theequilibrium is again attained. The end results will depend upon the rateat which the composition changes as a consequence of the segregation, ascompared to the rate with which atoms are transferred between the meltand the vapor, which in turn depends on the speed of the solidifyingprocess. Eventually a stationary state will be reached in which thecrystals attain the desired composition. As a consequence, this systemcontrols the composition of the melt, and consequently the solid, bycontrolling the pressure of one of the components of the melt in thevapor phase, stops decomposition of the melt, and fixes the distributioncoefficients of the dopants.

The inventive process is accomplished by means of a modified Bridgmantechnique utilizing a novel two-part furnace. Each part of the furnaceis provided with different isothermal temperature profiles. An elongatedgrowth tube has one part in which cadmium or tellurium is placed so thatit may be heated to a temperature in one of the isothermal temperatureportions. In this portion, the vapor pressure of the cadmium ortellurium is controlled. in another portion of the tube, cadmiumtelluride is positionedand heated to the temperature of the secondisothermal temperature portion, the second portion being at atemperature sufficient to create a melt from the cadmium telluride and,therefore, being higher than the temperature of the first isothermaltemperature portion.

Cadmium telluride utilized in the process of the present invention ispreferred to have a purity of at least 99.9999%. In the operation of theinventive process, the tempereature of the upper furnace containing thecadmium or tellurium of similar purity is adjusted so as to provide thedesired pressure of cadmium or tellurium vapor. This partial pressure ofthe one element controls the concentration of the cadmium and telluriumions in the melt in such a manner that the melt will be eitherstoichiometrically pure, cadmium rich, or tellurium rich. The tube ismaintained stationary for a period of time sufficient to obtainstability of the composition of the melt. Thereafter, the tube is movedslowly through the furnace so that the melt passes through thedecreasing temperature gradient and solidifies into a crystal. Theisothermal temperature profile for the cadmium or tellurium in the upperpart of the tube is sufficiently long so that the temperature thereofdoes not vary in order to insure retention of stability and the desiredcarrier concentration. After the last portion of the melt has beensolidified, the tube is slowly cooled in a controlled manner.

It is, therefore, an object of the present invention to provide desiredconductivity type and carrier concentration in Group ll-Vl, Group Ill-V,and Group lV-Vl compounds of the Table of Periodic Elements.

Another object is to provide P-type, N-type, and intrinsic conductivityin such compounds.

Other aims and objects as well as a more complete understanding of thepresent invention will appear from the following explanation ofexemplary embodi- 3 ments and the accompanying drawings thereof, inwhich:

FIG. 1 illustrates a schematic representation of a twopart furnace witha crystal growing tube therein for growth of the crystals of the presentinvention;

FIG. 2 schematically depicts the temperature curves of the furnaces;

FIG. 3 is a graph, not to scale, of the resistivity versustemperature-pressure curve for obtaining carrier concentration andresistivity of the crystals of the present invention; and

FIG. 4 schematically depicts the temperaturecomposition phase curve fora two-constituent compound.

Accordingly, a pair of furnaces and 12 are vertically placed one atopthe other or in tandem with a central opening 14 extending therethroughfor placement therein and movement of a crystal growing tube 16. Abarrier 17 is positioned in the tube to act as a collector of thevolatile component in case refluxing occurs. The temperature of furnace10 is controlled so as to provide a temperature curve 18 while thetemperature of furnace 12 is controlled to provide a temperature profile20. The temperature of furnace 10 is less than that of furnace 12 asshown in FIG. 2, thereby producing a temperature gradient 21therebetween. The temperature of furnace 12 is further controlled toprovide a decreasing temperature gradient 22. Furnace 10 is disposed tobe at a temperature to provide a vapor pressure equal to or greater thanthe minimum pressure at which sublimation and/or decomposition occurs toprevent sublimation and decomposition, while furnace 12 is heated toprovide a maximum temperature of above the melting point of thecompound.

Tube 16 is provided with a lower section 24 in which a charge of purell-Vl, Ill-V, or IV-Vl Group compound is placed. One constituent 28 ofthe compound is placed in a receptacle or reservoir 30 at anothersection 32 of tube 16 and secured thereto by one or more supports 34.The preferred purity of the compound and constituent is at least99.9999% in order to prevent the conductivity and type of material frombeing determined by the concentration and kind of impurities which wouldbe otherwise present. Section 24 of the tube is terminated by anucleation point 36 of any suitable shape, as is well known in the art.Support of section 24 may be provided or section 32 may have attached toit a rod 38 for rotating and lowering the tube through the furnace. Flatportion 39 of temperature curve 18 is maintained for a sufficiently longlength so that, when tube portion 24 moves into decreasing temperaturegradient 22, constituent 28 will remain in flat temperature portion 39.

In operation, a quantity of II-Vl, III-V, or IV-Vl Group compound, incompound or elemental form, which need not be stoichiometric, is placedin section 24 of tube 16 while the desired compound constituent isplaced within receptacle 30 of the tube. The tube is then sealed andevacuated to approximately 10 Torr, or partially filled or pressurizedwith an inert gas. The tube with its contained materials is then placedwithin the two-part furnace as shown in FIG. 1 and the temperatures offurnaces 10 and 12 are raised to provide a melt of compound 26 andvaporization of constituent 28. The temperature of constituent 28 atflat portion 39 is adjusted to provide a desired vapor pressure abovethe decomposition and/or sublimation pressure. As a consequence, athermodynamic equilibrium is soon established between vaporizedconstituent 28 and melt 26 because, if the melt is deficient in thevolatile component, such as cadmium, for this vapor pressure, then thecomponent is removed from the vapor and added to the melt, andconversely, if the melt is rich in the component, then it leaves themelt as a vapor. Thus, regardless of what the composition of the meltmay originally be, the composition can be reproducibly adjusted towhatever composition is desired by controlling the temperature of thevapor. For example, if constituent 28 comprises cadmium and compoundmelt 26 comprises cadmium telluride, an increase in the cadmiumconcentration in the melt will produce a correspondingly high electronconcentration therein. The opposite result is obtained with a telluriumconstituent as the tellurium pressure increases.

In the present invention, the pressure of the component in reservoir 30is changed by changing the temperature of furnace 10. For cadmiumtelluride, by increasing the cadmium pressure, and consequentlyeffecting a reduction in the tellurium pressure, more cadmium is addedto the system. Thus, the composition of cadmium telluride is controlled.Also, the same process would apply to tellurium. Adding tellurium wouldreduce the cadmium pressure, and consequently, the cadmium content.

Specifically, with reference to FIG. 4, wherein L is the liquidus phaseand S is the solidus phase of a twocomponent system comprisingcomponents A and B, for each point, x, on the liquidus-solidus interfacerepresented by curve 40, there is a corresponding pressure of thevolatile component A or B. If a certain vapor pressure of the componentis maintained over the melt, the system has a tendency to remain at thecomposition corresponding to the vapor pressure applied. Assolidification takes place, the segregation process tends to make themelt more and more concentrated with regard to the component present inexcess; however, the liquid then is no longer in equilibrium with thevapor, and a reaction between the vapor melt occurs until equilibrium isagain reached. The end results of these interactions depend on the rateat which the composition changes as a consequence of the segregation,compared to the rate with which atoms are transferred between the meltand the vapor. Eventually, a stationary state is reached in which thecrystals attain a composition somewhere between x' and m in theexistence regioncontained within curve 41.

Because the composition of cadmium telluride is a function of thechemical potential of one of the components when grown from the melt, itis desirable to grow crystals of cadmium telluride under conditionswhere the chemical potential of one of the components is controlled.This is accomplished with the modified Bridgman system or techniqueutilized in conjunction with elongated tube 16 so that cadmium ortellurium can be placed high in the tube in receptacle 30 where itsvapor pressure is controlled by furnace 10 operating significantly belowthe temperature of crystal furnace 12. Thus, the properties of thecompound are determined by the pressure of one of the components abovethe melt.

The modified Bridgman system noted above controls the composition of themelt and, consequently, the solid by controlling the pressure of one ofthe components of cadmium telluride over the melt, stops decompositionand/or sublimation of the melt, and controls the distributioncoefficients of the dopants.

The theory of equilibrium between. solid-liquid-gas discussed herein forthis system indicates that the liquid acts only as a transferring mediumbetween gas and solid phases and, consequently, the system may betreated as a gas-solid system with respect to defect chemistry. Inexplanation of this theory, it is helpful to assume a model for theliquid which is equivalent to the normal crystalline model of the solidexpressible as either of two types, a gas-like model and a crystallikemodel. In the gas-like model, the perfect liquid is assumed to consistof molecules and dissociation products of the molecules. Ionization mayoccur which leads to additional imperfections. In the crystal-likemodel, the liquid is regarded as a crystal with an exceptionally largeconcentration of the Schottky-Wagner or Frenkel type defects. Intrinsicexcitation and ionization are assumed to take place exactly as in asolid. Statistically both models are equivalent and give the sameresults.

Consequently, in the following explanation using cadmium telluride as anexample where cadmium is the A component and tellurium is the Bcomponent, where x indicates an uncharged atom, a is the defect inliquid caused by the addition ofCd; in the liquid, ,8 is the defect inliquid caused by removal of Cd from the liquid; g, l, and s are thegaseous, liquid, and solid phases, respectively, of the cadmiumtelluride components; P is pressure, and K is the equilibrium constant.Accordingly, Cd, :Cd, a, and for small concentrations of 1= (Md/ m) (1)Also, Cd; 2 Cd B, and for small concentrations of K2: IBH cdI- (2) Underthese conditions,

nm t'a Te (3) since CdTe I Cd rTe Accordingly,

[ MB] an or the composition of the liquid is a function of P In theequilibrium between liquid and solid, the transfer of atoms from onephase to the other as well as imperfections within each phase aredescribable as reactions. Transfer of electrons or ions need not beconsidered since both phases remain neutral. If such particles aretransferred, electrons and ions are transferred in equal concentrations,so that the final reactions may be expressed in terms of the transfer ofneutral atoms and vacancies. Consequently, let a be a defect caused byremoval of a cadmium atom from the solid. Then,

Cd, ,8 :1 Cd, b where Cd, and Ca, are unionized atoms in solid andliquid, respectively. For small concentrations of B and b,

X, (b/B) (5) Also, Te, a 2- Te, a,

4 (ale), (6)

and

l ll l ab (7) 6 With the aid of equations (4), (5), (6), and (7),

l ll l KZIK4KUB ub' The foregoing theory was simplified by assuming that(l the liquid is always homogeneous, (2) diffusion in the solid is slowand, therefore, the composition of. the solid does not change afterhaving grown from the liquid, (3) the gas phase equilibrium CdTe I Cd Teis reached quickly with cadmium telluride molecules not present in anysignificant amounts, (4) the distribution coefficients are constantswhich are independent of composition, and (5) the solid-liquiddistribution is adjusted quickly. In the modified Bridgman systemdescribed herein, the gas and solid phases are physically separated fromeach other by the liquid phase so that essentially the liquid phase actsas a communication medium since the composition of the liquid phase isdetermined by the pressure of one of the components of cadmiumtelluride.

The quantity of constituent 28 needed in reservoir 30 is not criticalsince the pressure is determined by the temperature of the reservoir andis independent of the amount of material in the reservoir. Of course, ifinsuf ficient material were placed in the reservoir, then obviously therequired pressure could not be obtained. The exact amount can bedetermined from a pressuretemperature-composition phase diagram;however, in actual practice, simply more constituent is used than isrequired.

As an aid to understanding the method by which a particular electricalcharacteristic, in terms of resistiw ity, and the carrier type of thecrystal is obtained, reference is made to FIG. 3. Curve 42 representsthe resistivity curve for cadmium telluride which is representative ofsome II-VI, III-V, or IV-VI Group compound crystals having either aP-type or an N-type nature. A maximum resistivity occurs at point 44 ofcurve 42. To the left of a line 46 passing through point 44, the crystalis P-type while to the right of line 46, the crystal is N- type. As thecrystal becomes more P-type or N-type, the resistivity decreases. Eachpoint of resistivity corresponding to the degree of P-type or N-typeconcentration is determined by the pressure of the crystal constituent.Thus, as the melt composition moves toward the right hand side, that is,toward the N-type area, so also does the solid composition move and theelectrical properties will accordingly vary. Furthermore, to obtain theparticular electrical characteristic, the com pound composition mustremain a constant. By using a reservoir supply, the melt composition canbe kept constant, thus insuring a constant solid composition. Forcadmium telluride and the cadmium constituent, as the temperature 18 offurnace 10 increases, the cadmium pressure also increases, therebyincreasing the conductivity. The same analysis is true when the cadmiumpressure is allowed to decrease toward the left of the figure in orderto obtain a P-type semiconductor material. The further to the left thecomposition moves in accordance with decreased cadmium pressure, thelower the resistivity becomes. Therefore, the temperature and pressureof the cadmium constituent are so controlled as to obtain a point justto the left or right of the highest resistivity characteristic at point44.

Therefore, the pressure of the compound constituent is closelycontrolled so as to obtain the desired resistivity. Initially, the tubeand its contained materials are maintained stationary within furnaces l0and 12 for a period of time sufficient to obtain stabilization ofcarcadmium telluride, so that nucleation tip 36 passes' through thedecreasing temperature gradient 22 in order to solidify the melt into asingle crystal as it passes through the liquid-solid plane of thedecreasing temperature gradient. The length of furnace l and its 10uniform temperature curve 18 is maintained long enough so that thetemperature, and therefore, the pressure of constituent 28 will not varyand thereby will not affect the constituent vapor pressure.

The following table lists examples of materials prepared by use of thedescribeid method:

the isothermal temperature portions and the decreasing temperaturegradient by said positioning and moving means, and

said second section having means for containing one constituent of theselected Group compound, said second section being placeable within andmovable only within the second of the isothermal temperature portions bysaid positioning and moving means said first and second containing meanshaving a specified distance therebetween,

the length of the second isothermal temperature portion being at leastas long as the combined length of the first isothermal temperatureportion and the decreasing temperature gradient and being at least aslong as the specified distance between said first and second containingmeans, and the said contain- Conductivity type of high resistivitymaterial was not practically possible to determine and was uncertain.Theorcticully. it is P-type butcot ld bqrt&r lfl;tzgc;jigs. it isintrinsic Although the invention has been described with reference to aparticular embodiment thereof, it should be realized that variouschanges and modifications may be made therein without departing from thespirit and scope of the invention.

What is claimed is: V

1. An apparatus for obtaining desired conductivity type and carrierconcentration in semiconductor crystal compounds selected from one ofthe lI-VI, lII-V,

and lV-VI Groups of the Periodic Table of Elements ing means of saidsecond tube section remaining within the second isothermal temperatureportion when the containing means of said first tube section movesthrough the first isothermal temperature portion and the decreasingtemperature gradient.

2. An apparatus as in claim 1 further including a reservoir securedwithin said second tube section for receiving the one constituent.

3. An apparatus as in claim 1 further including barrier means secured tothe interior of said tube within said spacing means and intermediatesaid first and second tube sections for collecting refluxed volatileconstituent.

4. A crystal growth tube closed from the external environment for use insynthesizing crystal materials comprising:

. g tgradien: terminating the fi of i a first section for nucleating thecrystal materials;

' e ma "F pot Ions an passmg a second section for receiving at leastsome materials through the crystallization temperature of the seto benucleated and spaced from and physically 2: 15 2232 23 t l th t b dabove said first section;

. mg a crys a grow u e m spacing means separating said first and secondsecmovmg said tube through said central opening 5 tions, and

P 3 tube f fi means for defining a barrier secured to the interior of anmeans or attac mg sectlons said tube within said spacing means,intermediate said first and second tube sections, and opening to- Saldfirst sectlon havmg means the wards said second section for collectingrefluxed lected Group compound, said first section being materiaLplaceable within and movable through the first of t

1. AN APPARATUS FOR OBTAINING DESIRED CONDUCTIVITY TYPE AND CARRIERCONCENTRATION IN SEMICONDUCTOR CRYSTAL COMPOUNDS SELECTED FROM ONE OFTHE II-VI, III-V, AND IV-VI GROUPS OF THE PERIODIC TABLE OF ELEMENTSCOMPRISING: A PAIR OF FURNACE MEANS PLACED IN TANDEM AND HAVING CENTRALOPENING MEANS EXTENDING THERETHROUGH, EACH OF SAID FURNACE MEANS HAVINGMEANS FOR PROVIDING ELONGATED ISOTHERMAL TEMPERATURE PORTIONS OFSPECIFIED LENGTHS AND FOR PROVIDING A DECREASING TEMPERATURE GRADIENTTERMINATING THE FIRST OF THE ISOTHERMAL TEMPERATURE PORTIONS AND PASSINGTHROUGH THE CRYSTALLIZATION TEMPERATURE OF THE SELECTED COMPOUND; MEANSFOR POSITIONING A CRYSTAL GROWTH TUBE IN AND MOVING SAID TUBE THROUGHSAID CENTRAL OPENING MEANS, SAID TUBE COMPRISING FIRST AND SECONDSECTIONS AND MEANS FOR ATTACHING SAID SECTIONS TOGETHER, SAID FIRSTSECTION HAVING MEANS FOR CONTAINING THE SELECTED GROUP COMPOUND, SAIDFIRST SECTION BEING PLACEABLE WITHIN AND MOVABLE THROUGH THE FIRST OFTHE ISOTHERMAL TEMPERATURE PORTIONS AND THE DECREASING TEMPERATUREGRADIENT BY SAID POSITIONING AND MOVING MEANS, AND SAID SECOND SECTIONHAVING MEANS FOR CONTAINING ONE CONSTITUENT OF THE SELECTED GROUPCOMPOUND, SAID SECOND SECTION BEING PLACEABLE WITHIN AND MOVABLE ONLYWITHIN THE SECOND OF THE ISOTHERMAL TEMPERATURE PORTIONS BY SAIDPOSITIONING AND MOVING MEANS SAID FIRST AND SECOND CONTAINING MEANSHAVING A SPECIFIED DISTANCE THEREBETWEEN, THE LENGTH OF THE SECONDISOTHERMAL TEMPERATURE PORTION BEING AT LEAST AS LONG AS THE COMBINEDLENGTH OF THE FIRST ISOTHERMAL TEMPERATURE PORTION AND THE DECREASINGTEMPERATURE GRADIENT AND BEING AT LEAST AS LONG AS THE SPECIFIEDDISTANCE BETWEEN SAID FIRST AND SECOND CONTAINING MEANS, AND THE SAIDCONTAINING MEANS OF SAID SECOND TUBE SECTION REMAINING WITHIN THE SECONDISOTHERMAL TEMPERATURE TURE PORTION WHEN THE CONTAINING MEANS OF SAIDFIRST TUBE SECTION MOVES THROUGH THE FIRST ISOTHERMAL TEMPERATUREPORTION AND THE DECREASING TEMPERATURE GRADIENT.
 1. An apparatus forobtaining desired conductivity type and carrier concentration insemiconductor crystal compounds selected from one of the II-VI, III-V,and IV-VI Groups of the Periodic Table of Elements comprising: a pair offurnace means placed in tandem and having central opening meansextending therethrough, each of said furnace means having means forproviding elongated isothermal temperature portions of specified lengthsand for providing a decreasing temperature gradient terminating thefirst of the isothermal temperature portions and passing through thecrystallization temperature of the selected compound; means forpositioning a crystal growth tube in and moving said tube through saidcentral opening means, said tube comprising first and second sectionsand means for attaching said sections together, said first sectionhaving means for containing the selected Group compound, said firstsection being placeable within and movable through the first of theisothermal temperature portions and the decreasing temperature gradientby said positioning and moving means, and said second section havingmeans for containing one constituent of the selected Group compound,said second section being placeable within and movable only within thesecond of the isothermal temperature portions by said positioning andmoving means said first and second containing means having a specifieddistance therebetween, the length of the second isothermal temperatureportion being at least as long as the combined length of the firstisothermal temperature portion and the decreasing temperature gradientand being at least as long as the specified distance between said firstand second containing means, and the said containing means of saidsecond tube section remaining within the second isothermal temperatureportion when the containing means of said first tube section movesthrough the first isothermal temperature portion and the decreasingtemperature gradient.
 2. An apparatus as in claim 1 further including areservoir secured within said second tube section for receiving the oneconstituent.
 3. An apparatus as in claim 1 further including barriermeans secured to the interior of said tube within said spacing means andintermediate said first and second tube sections for collecting refluxedvolatile constituent.
 4. A CRYSTAL GROWTH TUBE CLOSED FROM THE EXTERNALENVIRONMENT FOR USE IN SYNTHESIZING CRYSTAL MATERIALS COMPRISING: AFIRST SECTION FOR NUCLEATING THE CRYSTAL MATERIALS; A SECOND SECTION FORRECEIVING AT LEAST SOME MATERIAL TO BE NUCLEATED AND SPACED FROM ANDPHYSICALLY ABOVE SAID FIRST SECTION; SPACING MEANS SEPARATING SAID FIRSTAND SECOND SECTIONS; AND MEANS FOR DEFINING A BARRIER SECURED TO THEINTERIOR OF SAID TUBE WITHIN SAID SPACING MEANS, INTERMEDIATE SAID FIRSTAND SECOND TUBE SECTIONS, AND OPENING TOWARDS SAID SECOND SECTION FORCOLLECTING REFLUXED MATERIAL.